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SubjectRE: [PATCH] rcu: Fix race in set and clear TICK_DEP_BIT_RCU_EXP bitmask
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> For the kernel bulit with CONFIG_NO_HZ_FULL enabled and the following 
> cpus is nohz_full cpus:
>
> CPU1 CPU2
> rcu_report_exp_cpu_mult synchronize_rcu_expedited_wait
> acquires rnp->lock mask = rnp->expmask;
> for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> rnp->expmask = rnp->expmask & ~mask; rdp = per_cpu_ptr(&rcu_data, cpu1);
> for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> rdp = per_cpu_ptr(&rcu_data, cpu1);
> if (!rdp->rcu_forced_tick_exp)
> continue; rdp->rcu_forced_tick_exp = true;
>
> tick_dep_set_cpu(cpu1, TICK_DEP_BIT_RCU_EXP);
>
> In the above scenario, after CPU1 reported the quiescent state, CPU1
> misses the opportunity to clear the TICK_DEP_BIT_RCU_EXP bitmask, it
> will not be cleared until the next expedited grace period starts and
> the CPU1 quiescent state is reported again. during this window period,
> the CPU1 whose tick can not be stopped, if CPU1 has only one runnable
> task and this task has aggressive real-time response constraints, this
> task may have one of the worst response times.
>
> Therefore, this commit add rnp->lock when set TICK_DEP_BIT_RCU_EXP
> bitmask to fix this race.
>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>
>Good eyes, thank you!!!
>
>Queued for testing and further review as follows, as always, please check for errors.
>

It looks more clear now, thank you!

Thanks
Zqiang

> Thanx, Paul
>
>------------------------------------------------------------------------

commit acfe689f2e473fb59b6d2c95af5fe36198bb9a84
Author: Zqiang <qiang1.zhang@intel.com>
Date: Tue Dec 20 19:25:20 2022 +0800

rcu: Fix set/clear TICK_DEP_BIT_RCU_EXP bitmask race

For kernels built with CONFIG_NO_HZ_FULL=y, the following scenario can result
in the scheduling-clock interrupt remaining enabled on a holdout CPU after
its quiescent state has been reported:

CPU1 CPU2
rcu_report_exp_cpu_mult synchronize_rcu_expedited_wait
acquires rnp->lock mask = rnp->expmask;
for_each_leaf_node_cpu_mask(rnp, cpu, mask)
rnp->expmask = rnp->expmask & ~mask; rdp = per_cpu_ptr(&rcu_data, cpu1);
for_each_leaf_node_cpu_mask(rnp, cpu, mask)
rdp = per_cpu_ptr(&rcu_data, cpu1);
if (!rdp->rcu_forced_tick_exp)
continue; rdp->rcu_forced_tick_exp = true;
tick_dep_set_cpu(cpu1, TICK_DEP_BIT_RCU_EXP);

The problem is that CPU2's sampling of rnp->expmask is obsolete by the
time it invokes tick_dep_set_cpu(), and CPU1 is not guaranteed to see
CPU2's store to ->rcu_forced_tick_exp in time to clear it. And even if
CPU1 does see that store, it might invoke tick_dep_clear_cpu() before
CPU2 got around to executing its tick_dep_set_cpu(), which would still
leave the victim CPU with its scheduler-clock tick running.

Either way, an nohz_full real-time application running on the victim
CPU would have its latency needlessly degraded.

Note that expedited RCU grace periods look at context-tracking
information, and so if the CPU is executing in nohz_full usermode
throughout, that CPU cannot be victimized in this manner.

This commit therefore causes synchronize_rcu_expedited_wait to hold
the rcu_node structure's ->lock when checking for holdout CPUs, setting
TICK_DEP_BIT_RCU_EXP, and invoking tick_dep_set_cpu(), thus preventing
this race.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 249c2967d9e6c..7cc4856da0817 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -594,6 +594,7 @@ static void synchronize_rcu_expedited_wait(void)
struct rcu_data *rdp;
struct rcu_node *rnp;
struct rcu_node *rnp_root = rcu_get_root();
+ unsigned long flags;

trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
jiffies_stall = rcu_exp_jiffies_till_stall_check();
@@ -602,17 +603,17 @@ static void synchronize_rcu_expedited_wait(void)
if (synchronize_rcu_expedited_wait_once(1))
return;
rcu_for_each_leaf_node(rnp) {
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
mask = READ_ONCE(rnp->expmask);
for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
rdp = per_cpu_ptr(&rcu_data, cpu);
if (rdp->rcu_forced_tick_exp)
continue;
rdp->rcu_forced_tick_exp = true;
- preempt_disable();
if (cpu_online(cpu))
tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
- preempt_enable();
}
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
j = READ_ONCE(jiffies_till_first_fqs);
if (synchronize_rcu_expedited_wait_once(j + HZ))
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